1. Field of the Invention
The present invention relates to a propulsion apparatus for a vessel which comprises a main screw and a POD propeller, and in particular comprises a push type POD propeller.
2. Description of the Related Art
Recently, in a propulsion apparatus for a vessel, installation of a POD propeller rearward of a main screw has been proposed in order to add a further propulsive force when the propulsive force generated by the main screw is insufficient.
FIG. 5 shows a conventionally proposed example of a propulsion apparatus for a vessel. In FIG. 5, reference numeral 1 denotes a rear portion of the bottom of the vessel, reference numeral 2 denotes a main screw for generating a main propulsive force for moving the vessel, and reference numeral 10 denotes a push type POD propeller. The main screw is provided so as to rotate by a driving force generated by an engine such as a diesel engine (not shown).
The push type POD propeller 10 used in the above propulsion apparatus is composed of a casing 11, a POD screw 12, a strut 13, and a supporting rod 14.
The casing 11 is formed in a cylindrical shape and the POD screw 12 is provided rearward thereof. The POD screw 12 generates a propulsive force by rotating, and an electric motor (not shown) for driving the POD screw 12 is provided in the casing 11.
The strut 13 has a wing-shaped section and is provided on the upper side of the casing 11, and the supporting rod 14 which acts as a rotational axis of the POD propeller 10 stands upright from the upper end of the strut 13. The supporting rod 14 is connected to a driving mechanism in a hull (not shown), and as a result, the POD propeller 10 is rotatably installed on the rear portion of the bottom of the vessel 1 through the supporting rod 14.
The vessel having the propulsion apparatus for a vessel is moved by the propulsive force obtained by only rotating the main screw 2 or the POD screw 12, or rotating both the main screw 2 and the POD screw 12. Furthermore, when turning the POD propeller 10 around the supporting rod 14, the strut 13 functions as a rudder and a steering force is generated, and as a result; the vessel can be turned.
However, in the conventional propulsion apparatus for a vessel, since the POD propeller 10 is provided rearward of the main screw 2, the propulsive force obtained by the main screw 2 is reduced by the effect of a vortex (hereinafter called xe2x80x9chub vortexxe2x80x9d) generated on a front end portion of the casing 11 by the main screw 2 itself, and the propulsion efficiency of the main screw 2 is decreased.
Furthermore, when the vessel is moved, a rotating flow remains in a slipstream of the main screw 2. This means that the energy given to the main screw 2 is partly consumed as energy for generating the rotating flow, and a problem occurs in that the propulsion energy of the main screw 2 is decreased.
The present invention is provided in consideration of the above circumstances, and an object of the present invention is to improve the propulsion efficiency of the propulsion apparatus for a vessel by reducing the hub vortex or by utilizing the energy for the rotating flow generated by the main screw.
In the present invention, the following features are provided in order to solve the above problems.
A first aspect of the propulsion apparatus for a vessel of the present invention comprises a main screw and a push type POD propeller which is provided rearward of the main screw a. A plurality of grooves, which extend along the flow directions of a hub vortex generated by the main screw, are provided on a front end portion of a casing of the POD propeller.
According to the propulsion apparatus for a vessel having the above-described features, the hub vortex generated by the main screw rearward of the main screw is weakened by diffusing the hub vortex along the grooves which are provided on the front end portion of the casing of the POD propeller. Therefore, the propulsion efficiency of the propulsion apparatus for a vessel is improved.
A second aspect of the propulsion apparatus for a vessel of the present invention comprises a main screw and a push type POD propeller which is provided rearward of the main screw, wherein a plurality of stator fins are provided on a front end portion of a casing of the POD propeller.
According to the propulsion apparatus for a vessel having the above-described features, the rotating flow generated by the main screw rearward of the main screw is converted to a propulsive force by the stator fins which are provided on the front end portion of the casing of the POD propeller. Therefore, the propulsion efficiency of the propulsion apparatus for a vessel is improved.
A third aspect of the propulsion apparatus for a vessel of the present invention comprises a main screw and a push type POD propeller which is provided rearward of the main screw, wherein the main screw and POD propeller have the same axes, and a hub portion of the main screw and a front end portion of the POD propeller form a continuous rough spindle shape.
According to the propulsion apparatus for a vessel, having the above-described features, the rotating flow generated by the main screw rearward of the main screw is removed to the outside along the surfaces of the hub portion of the main screw and the front end portion of the POD propeller which form the continuous rough spindle shape. Therefore, the generation of a hub vortex is prevented, and the propulsion efficiency of the propulsion apparatus for a vessel is improved.
A fourth aspect of the propulsion apparatus for a vessel of the present invention comprises a main screw and a push type POD propeller which is provided rearward of the main screw, wherein at least a pair of fins having wing-shaped sections are provided on both sides of a casing of the POD propeller.
According to the propulsion apparatus for a vessel having the above-described features, the rotating flow generated by the main screw rearward of the main screw is converted to a propulsive force by the fins which are provided on both sides of the casing of the POD propeller. Therefore, the propulsion efficiency of the propulsion apparatus for a vessel is improved.
In this case, it is preferable that a leading edge portion of each fin be twisted along the direction of the water flow which is generated by the main screw.